This invention relates to aqueous coating compositions based on two-component polyureas and to a process for their production.
It is generally desirable to use water based coatings instead of organic solvent based coatings for the sake of environmental considerations. However, the use of water instead of organic solvents in two-component isocyanate reactive coating compositions based on polyisocyanates containing free isocyanate groups has been hampered by the fact that isocyanate groups react not only with isocyanate reactive groups in the intended crosslinking reaction, but also with the water. Therefore, in a composition including a polyisocyanate, isocyanate reactive groups, and water, the isocyanate/water reaction takes place with formation of urea and carbon dioxide. This reaction not only fails to achieve the intended crosslinking of the isocyanate reactive groups, but also results in gassing or foaming of the composition due to the formation of carbon dioxide.
It was disclosed in U.S. Pat. No. 5,075,370 that it is possible to produce aqueous, two-component polyurethane coating compositions by using neutralized, acid group-containing polyhydroxy polymers, i.e. polyhydroxypolyacrylates, as dispersing agents for polyisocyanates containing free isocyanate groups. The polyisocyanates containing free isocyanate groups are emulsified in the aqueous, anionic polymer solution or dispersion. The coating compositions according to that patent are said to have a pot life of several hours and cure by way of isocyanate/hydroxyl group reaction to form polyurethane linkages.